1. Field of the Invention
The present invention relates to a camera having an auto-focus control system.
2. Description of the Prior Art
Generally, an auto-focus control camera inevitably includes a step of operation for automatically adjusting the focus of camera between the moment of the depression of the shutter button and the initiation of the exposure, and such a step of automatic focus adjusting operation requires an appreciable amount of time. Therefore, when compared with conventional cameras having no auto-focus system, in which the focus adjustment is manually practiced prior to the depression of the shutter button, the auto-focus control camera requires a greater time from the moment when the shutter button is depressed until the initiation of the exposure.
Furthermore, in the case of an auto-focus control camera equipped with a motor drive system that automatically winds up the film to successively carry out a plurality of photographing operations during the depression of the shutter button, the maximum speed of frame advance, or the minimum period between the successive shots, is restricted by the time necessary to carry out the automatic focus adjusting operation.
Further, if the time necessary to carry out the auto-focus operation varies depending on the conditions, such as the brightness of the subject to be photographed and the amount of required lens shift, the time necessary to carry out the auto-focus operation would greatly interfere with the desired frame advance speed. Particularly when the means for obtaining information necessary to shift the lens is formed by an array of photoelectric sensors, the time necessary to carry out the operation for obtaining information of the value of shifting the lens varies greatly on the brightness of the subject such that the auto-focus adjusting operation takes a longer time as the brightness of the subject to be photographed becomes darker.
For example, when the array of photoelectric sensors is formed by a CCD element that can detect a pattern of image of the subject, its response speed depends greatly on the brightness of the subject. An example of such an array of photoelectric sensors is disclosed in detail in U.S. Pat. No. 4,185,191 to N. L. Stauffer. Furthermore, such an array of sensors must be coupled with a calculation circuit which usually proceeds calculation in digital form. Thus, an analog-to-digital converter must be inserted between the array of sensors and the calculation circuit. The employment of these circuits in longer time before the auto-focus information obtaining operation is completed. For example, the auto-focus information obtaining operation for the bright subject takes several 10 ms, whereas that for the less bright subject takes several 100 ms.
Therefore, there has been a problem of employing an auto-focus control system defined by the array of photoelectric sensors in a camera because of such a time lag between the shutter button depression and actual exposure. This problem is much serious for the cameras equipped with the motor drive.